The disclosure relates generally to downhole tools. More particularly, the disclosure relates to downhole oscillation systems for inducing axial oscillations in drill strings during drilling operations. Still more particularly, the disclosure relates to shock tools that directly and efficiently convert cyclical pressure pulses in drilling fluid into axial oscillations.
Drilling operations are performed to locate and recover hydrocarbons from subterranean reservoirs. Typically, an earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
During drilling, the drillstring may rub against the sidewall of the borehole. Frictional engagement of the drillstring and the surrounding formation can reduce the rate of penetration (ROP) of the drill bit, increase the necessary weight-on-bit (WOB), and lead to stick slip. Accordingly, various downhole tools that induce vibration and/or axial reciprocation may be included in the drillstring to reduce friction between the drillstring and the surrounding formation. One such tool is an oscillation system, which typically includes an pressure pulse generator and a shock tool. The pressure pulse generator produces pressure pulses in the drilling fluid flowing therethrough and the shock tool converts the pressure pulses in the drilling fluid into axial reciprocation. The pressure pulses created by the pressure pulse generator are cyclic in nature. The continuous stream of pressure peaks and troughs in the drilling fluid cause the shock tool to cyclically extend and retract telescopically at the pressure peak and pressure trough, respectively. A spring is usually used to induce the axial retraction during the pressure trough.